Posted by Constantine on Thursday, April 18, 2013 at 3:45pm.
Hi Dr. Bob. Could you please take a look at the question below and my work? I'm not sure how to get the correct answer for this question. I would greatly appreciate your help!
How many of the following molecules are polar?
BrCl3
CS2
SiF4
SO3
A) 1
B) 2
C) 3
D) 4
E) 0
The correct answer for this question is A) which is 1. However I could not find a polar molecule among the ones given.
BrCl3: the molecule has a Tshaped molecular geometry which could make it polar, but the BrCl electronegativity difference is 0.2 which makes it a covalent bond.
CS2: the molecule has a linear geometry and the electronegativity difference between C and S is 0, so this one is not polar either.
SiF4: the molecule has a tetrahedral molecular geometry so even though the SiF electronegativity large and the bond is polar covalent, the bonds cancel each other out.
SO3: the molecule has a trigonal planar molecular geometry so the SO bonds cancel each other out.
Could you please let me know which molecule is polar and why?

Chemistry  molecular polarity  DrBob222, Thursday, April 18, 2013 at 10:00pm
Your reasoning is correct for SiF4, SO3 and CS2.
BrCl3 is the polar molecule. Did you draw the Lewis structure? I can't draw it on this forum but if you try you will note that there are 28 electrons. If you place them around the Br atom, you must have two lone pairs on the central atom. The Tshape should tell you it is polar; I like to compare electronic geometries.

Chemistry  molecular polarity  Constantine, Thursday, April 18, 2013 at 11:00pm
Thank you so much. I should've ignored the electronegativity factor.
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